The effect of delivery mechanisms on the uptake of bed net re-impregnation in Kilifi District, Kenya.

The results of recently completed trials in Africa of insecticide-treated bed nets (ITBN) offer new possibilities for malaria control. These experimental trials aimed for high ITBN coverage combined with high re-treatment rates. Whilst necessary to understand protective efficacy, the approaches used to deliver the intervention provide few indications of what coverage of net re-treatment would be under operational conditions. Varied delivery and financing strategies have been proposed for the sustainable delivery of ITBNs and re-treatment programmes. Following the completion of a randomized, controlled trial on the Kenyan coast, a series of suitable delivery strategies were used to continue net re-treatment in the area. The trial adopted a bi-annual, house-to-house re-treatment schedule free of charge using research project staff and resulted in over 95% coverage of nets issued to children. During the year following the trial, sentinel dipping stations were situated throughout the community and household members informed of their position and opening times. This free re-treatment service achieved between 61-67% coverage of nets used by children for three years. In 1997 a social marketing approach, that introduced cost-retrieval, was used to deliver the net re-treatment services. The immediate result of this transition was that significantly fewer of the mothers who had used the previous re-treatment services adopted this revised approach and coverage declined to 7%. The future of new delivery services and their financing are discussed in the context of their likely impact upon previously defined protective efficacy and cost-effectiveness estimates.

PIP: Recent trials of insecticide-treated bednets (ITBN) in Africa sought to achieve high ITBN coverage together with high net retreatment rates. Following the completion of a randomized, controlled trial on the coast of Kenya, a series of delivery strategies were used to continue net retreatment in the area. Adherence to a free bi-annual, house-to-house retreatment schedule resulted in a more than 95% coverage of nets issued to children. During the year following the trial, sentinel dipping stations were situated throughout the community and household members informed of their locations and opening times. More than 85% of bednets were re-impregnated between October 1993 and October 1995, and 61-67% coverage of nets used by children for 3 years after the free retreatment service was launched beginning in 1996. The introduction of a social marketing approach at sentinel sites in 1997 to retrieve some of the costs of the net retreatment services caused coverage to drop to 7.1% among children still resident in the study area who had had nets since 1993. The future of new delivery services and their financing are discussed with regard to their likely impact upon previously defined protective efficacy and cost-effectiveness estimates.

Vector-related case-control study of severe malaria in Kilifi District, Kenya.

A case-control study examined vector-related and environmental parameters associated with severe malaria in Kilifi District along the coast of Kenya. Over an 11-month period, 119 children identified with severe malaria infections at the Kilifi District Hospital were matched by age with control children who reported to the outpatient clinic with nonsevere infections. Intensive mosquito sampling was done in each of the case-control houses over a four-day period, beginning within a week of index case admission. A total of 109 environmental, demographic, behavioral, and animal husbandry variables were characterized for each household. Vector species (Anopheles gambiae s.l. and An. funestus) were detected in 40.1% and 36.1% of case and control houses, respectively. The relative abundance of vectors in individual houses was stable over the two-week resampling periods (r = 0.9). Both the overall abundance of anopheline mosquitoes (odds ratio [OR] = 1.5) and P. falciparum sporozoite rates (OR = 1.5) were not significantly different between case and control houses. In a matched analysis, 11 of 109 house variables associated significantly with severe malaria were also associated with vector abundance, as determined by chi-square linear trend analysis. Under conditions of year-round, low-level transmission on the coast of Kenya, the risk of severe disease in children is multifactorial and not governed strictly by transmission intensity or environmental heterogeneity affecting vector abundance and distributions. This suggests that current interventions that appear to be achievable only in areas where transmission is already low to moderate should be appropriate. However, such interventions should be monitored so that inappropriate and possibly disastrous control activities can be avoided in Africa.

Midgut bacteria in Anopheles gambiae and An. funestus (Diptera: Culicidae) from Kenya and Mali.

Field studies in Kenya and Mali investigated the prevalence of bacteria in the midguts of malaria vectors, and the potential relationship between gram-negative bacteria species and Plasmodium falciparum sporozoites. Midguts were dissected from 2,430 mosquitoes: 863 Anopheles funestus Giles and 1,037 An. gambiae s.l. Giles from Kenya, and 530 An. gambiae s.l. from Mali. An. funestus had a higher prevalence of gram-negative bacteria (28.5%) compared with An. gambiae collected in Kenya and Mali (15.4 and 12.5%, respectively). Twenty different genera of bacteria were identified by gas chromatography from 73 bacterial isolates from mosquito midguts. Pantoea agglomerans (Enterobacter agglomerans) was the most common species identified. There was no association between gram-negative bacteria in the midgut and P. falciparum sporozoites in field-collected An. gambiae s.l. and An. funestus. However, An. funestus females that harbored gram positive bacteria were more likely to be infected with sporozoites compared with those with no cultivable bacteria or gram negative bacteria in their midguts. Habitat-related variation in the prevalence of diverse types of bacteria in mosquitoes could influence malaria parasite development in mosquitoes and corresponding sporozoite prevalence.

The impact of permethrin-impregnated bednets on malaria vectors of the Kenyan coast.

The effects of introducing permethrin-impregnated bednets on local populations of the malaria vector mosquitoes Anopheles funestus and the An.gambiae complex was monitored during a randomized controlled trial at Kilifi on the Kenyan coast. Pyrethrum spray collections: inside 762 households were conducted between May 1994 and April 1995 after the introduction of bednets in half of the study area. All-night human bait collections were performed in two zones (one control and one intervention) for two nights each month during the same period. PCR identifications of An.gambiae sensu lato showed that proportions of sibling species were An.gambiae sensu stricto > An.merus > An.arabiensis. Indoor-resting densities of An.gambiae s.l. and the proportion of engorged females decreased significantly in intervention zones as compared to control zones. However, the human blood index and Plasmodium falciparum sporozoite rate remained unaffected. Also vector parous rates were unaltered by the intervention, implying that survival rates of malaria vectors were not affected. The human-biting density of An.gambiae s.l., the predominant vector, was consistently higher in the intervention zone compared to the control zone, but showed 8% reduction compared to pre-intervention biting rates-versus 94% increase in the control zone. Bioassay, susceptibility and high-performance liquid chromatography results all indicated that the permethrin content applied to the nets was sufficient to maintain high mortality of susceptible vectors throughout the trial. Increased rates of early outdoor-biting, as opposed to indoor-biting later during the night, were behavioural or vector composition changes associated with this intervention, which would require further monitoring during control programmes employing insecticide-treated bednets.

Relationships between Plasmodium falciparum transmission by vector populations and the incidence of severe disease at nine sites on the Kenyan coast.

The transmission of Plasmodium falciparum was studied in relation to the incidence of severe malaria infections at nine sites in the Kilifi District in Kenya. Intensive mosquito sampling during a one-year period yielded Anopheles gambiae s. l., An. funestus, An. coustani, An. squamosus, An. nili, and An. pharoensis. Anopheles gambiae s.l. was the predominant vector, comprising 98.4% of the total anophelines collected. Overall, 3.5% of 2,868 An. gambiae s.l. collected indoors and 0.8% of 261 collected outdoors contained P. falciparum sporozoites. Transmission was detected during 10 months, with peak periods from June to August and December to January. In eight of the nine sites, entomologic inoculation rates (EIRs) averaged only four infective bites per year (range 0-18); an annual EIR of 60 was measured for the site with the highest intensity of transmission. The incidence of severe malaria infections, ranging from 8.6 to 38.1 per 1,000 children (0-4 years), was not associated with EIRs. At these sites on the coast of Kenya, a high incidence of severe disease occurs under conditions of very low levels of transmission by vector populations. With respect to conventional approaches for vector control in Africa, decreases in transmission, even to levels barely detectable by standard approaches, may not yield corresponding long-term reductions in the incidence of severe disease.

Evaluation of light traps for sampling anopheline mosquitoes in Kilifi, Kenya.

Anopheline mosquitoes were sampled inside houses, where residents slept under untreated bednets, by CDC light traps and human-biting catches to evaluate light traps as a means for determining human exposure to malaria vectors in Kilifi District, Kenya. Mosquitoes were sampled during 2 all-night collections by light traps and one all-night biting catch in a series of 262 houses. Collections yielded 1,721 Anopheles gambiae s.l. and 46 An. funestus, and 60.3% of the houses were negative for anophelines. There was a significant correlation in numbers of An. gambiae s.l. captured by light traps and human-biting collections (r = 0.64), but light traps were biased and underestimated An. gambiae s.l. abundance. This bias increased with increasing mosquito abundance. In addition, the proportion of An. gambiae s.l. infected by Plasmodium falciparum was 2.3-fold higher in light traps than in human-biting collections. Along the coastal zone of Kenya where vector abundance is low, light traps do not provide an adequate estimate of man-vector contact when such information is required at the household level in epidemiological studies of malaria parasite transmission.

Low-level Plasmodium falciparum transmission and the incidence of severe malaria infections on the Kenyan coast.

The transmission of Plasmodium falciparum was studied in relation to the incidence of severe malaria infections at Sokoke and Kilifi town, Kilifi District, Kenya. Intensive mosquito sampling during a one-year period yielded Anopheles gambiae s.l., An. funestus, and An. coustani. Anopheles gambiae s.l. was the predominant vector, comprising 87.9% and 97.9% of the total anophelines collected in Sokoke and Kilifi town, respectively. The proportion of An. gambiae s.l. with P. falciparum sporozoite infections was 4.1% (20 of 491) in Sokoke and 2.2% (3 of 138) in Kilifi town; no infections were detected in An. funestus or in An. coustani. Entomologic inoculation rates indicated that residents were exposed to only 8.0 infective bites per year in Sokoke and 1.5 in Kilifi town. Transmission was detected during only six months in Sokoke and three months in Kilifi town despite low-level, year-round vector activity. The yearly incidence of severe P. falciparum infections in children, 1-4 years of age was 24.1 per 1,000 in Sokoke and 4.2 per 1,000 in Kilifi town. Monthly patterns of transmission corresponded closely with the incidence of severe infections. At these sites on the coast of Kenya, the spatial and temporal incidence of severe malaria infections is associated with low-level P. falciparum transmission by vector populations.

Bloodfeeding behavior of Anopheles gambiae s.l. and Anopheles funestus in Kilifi District, Kenya.

Blood meal samples were tested by ELISA for 534 Anopheles gambiae s.l. and 76 Anopheles funestus collected from 25 sites in Kilifi District, Kenya. Human IgG was detected in 94.4% of the An. gambiae s.l. and in 90.8% of the An. funestus. No samples were positive for cow and only a few were positive for goat. Both species fed predominantly on humans irrespective of host availability. At these sites on the Kenyan coast, the high degree of human-feeding by malaria vectors contributes to the efficiency of malaria parasite transmission and the high incidence of severe malaria.

Plasmodium falciparum infection rates in malaria vectors at two ecologically district sites in Kilifi Districts; Kenya

Transmission of Plasmodium falciparum was studied for one year at two ecologically different sites in Kilifi district. Anopheline mosquitoes were collected by three sampling methods and tested for P.falciparum sporozoites by ELISA. Anopheles gambiae s.l. was the predominant vector. Plasmodium falciparum sporozoite rates were 4.07 (20/491) and 2.17 (3/138) for An.gambiae s.l. at Sokoke and Kilifi town respectively. No sporozoite infections were detected in An.funestus or An.coustani. Transmission was year-round with highest levels occurring after the long rains (June to September). Entomological inoculation rates indicated taht each person could receive 8 infcetive bites per year at Sokoke; compared with 1.3 infective bites per year at Kilifi town. Our studies indicate taht the public health problem of malaria in Kilifi district is associated with extremely low levels of P.falciparum transmission by vector populations